At present, optical communication systems most commonly use fiber amplifiers based on Erbium-doped fibers. Characteristics of such amplifiers limit their performance to wavelength bandwidth 1525 nm to 1560 nm; when modified, their functionality can be expanded to bandwidth 1570 to 1600 nm. Growing demands for volumes of transmitted information however require exploitation of other wavelength bands where optical fibers show low losses. Therefore, in the last decade and especially in connection with development of powerful semiconductor laser diodes, the attention has been focused on utilization of Stimulated Raman Scattering for scattered amplification directly in the transmission fiber. Such fiber amplifiers are called Raman fiber amplifiers.
Advantage of Raman fiber amplifiers is that they use transmission medium as an active environment. This means that in contrary to optical amplifiers based on fibers doped by rare earth elements they do not need special active fiber, which would predetermine their spectral range of possible application. Another advantage is that the spectral area, in which the Raman fiber amplifiers can be used, is flexible and is determined only by so-called Raman downshift between pump and signal waves; this downshift for silica fibers is roughly 12 THz. Raman fiber amplifiers bandwidth is moreover defined by the number of pump wavelengths and therefore it is not determined by spectral properties of chosen special fiber doped by rare earth elements. Using 4 pump wavelengths with suitable spectral deployment allows to achieve bandwidth broader than 80 nm, which is impossible with commonly used Erbium-doped fiber amplifiers.
As a source of pump radiation, current broadband Raman fiber amplifiers use frequency-multiplexed power laser diodes operating in continuous mode. In order to suppress pump waves' amplitude noise transfer to signal waves, so-called contra-directional pumping of Raman fiber amplifiers is usually used. This means that pump waves spread from the opposite end of the transmission fiber than signal waves. Suitable selection of wavelengths and power rating of individual pump diodes allows to achieve rather flat spectral response to signal amplification within the required frequency bandwidth. Shortcoming of these amplifiers is that transfers of energy between individual pump waves occur due to Raman interactions between them, causing higher-frequency pump waves to amplify lower-frequency pump waves, which as a result protrude with higher power to the other, signal, side of the transmission fiber. Resulting effect is that the optical signal-to-noise ratio almost linearly improves with increasing signal wavelength. At broadband Raman amplifiers, optical signal-to-noise ratio difference between signals at both ends of amplification spectrum may reach even several decibels despite suitable selection of pump waves frequencies and power ratings, which allows achieving rather flat spectral response to amplification with maximum ripple lower than few tenths of decibel. When multiple Raman fiber amplifiers are cascaded, the effect of linearly improved optical signal-to-noise ratio adds up and the difference between signals at the opposite ends of spectrum may be unacceptably high and cause higher error rates in transfer of information at the shortwave spectrum side.